U.S. patent number 6,002,894 [Application Number 09/039,845] was granted by the patent office on 1999-12-14 for single-pass fusing of sheet-fed multi-layer duplex copies.
This patent grant is currently assigned to Agfa-Gevaert. Invention is credited to Marc De Niel, Serge Tavernier, Luc Van Aken, Luc Van Goethem.
United States Patent |
6,002,894 |
De Niel , et al. |
December 14, 1999 |
**Please see images for:
( Certificate of Correction ) ** |
Single-pass fusing of sheet-fed multi-layer duplex copies
Abstract
A fusing station (25) of an electrographic apparatus fixes in a
single pass a duplex resinous powder color image (8, 18) to a
support material in sheet-form (9) as the sheet is moved over a
predetermined path (7). The station comprises two heated fixing
rollers (1, 11), rotating in contact with each other, driving means
to rotate the fixing rollers, pressing means for applying a meshing
force between the fixing rollers, heating sources (4, 14) which
have substantially identical characteristics. Both fixing rollers
comprise a heat conducting core (3, 13) and a resilient covering
(2, 12) which by the pressure between both rollers forms a heating
nip. A symmetrical fixing operation on both sides of the sheet is
provided. Hereto, the fixing rollers have a substantially identical
construction, are positioned symmetrically to the path of the sheet
and rotate synchronously to the advancement of the sheet.
Inventors: |
De Niel; Marc (Hove,
BE), Tavernier; Serge (Lint, BE), Van Aken;
Luc (Kuringen, BE), Van Goethem; Luc
(Sint-Gillis-Waas, BE) |
Assignee: |
Agfa-Gevaert (Mortsel,
DE)
|
Family
ID: |
8228111 |
Appl.
No.: |
09/039,845 |
Filed: |
March 16, 1998 |
Foreign Application Priority Data
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Mar 14, 1997 [EP] |
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97200780 |
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Current U.S.
Class: |
399/69; 219/216;
219/469; 399/328; 399/330; 399/333 |
Current CPC
Class: |
G03D
15/022 (20130101); G03G 15/2007 (20130101); G03G
2215/2074 (20130101); G03G 2215/2093 (20130101); G03G
2215/2083 (20130101) |
Current International
Class: |
G03G
15/20 (20060101); G03D 15/02 (20060101); G03G
015/20 () |
Field of
Search: |
;399/325,327,328,330,333,335,337,336,338,69,67
;219/216,469,470 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0435516 |
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Jul 1991 |
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EP |
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0486723 |
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May 1992 |
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EP |
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2102870 |
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Aug 1972 |
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DE |
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2197619 |
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May 1988 |
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GB |
|
Other References
Patent Abstracts of Japan, vol. 15, No. 430(P-1270), Pub. No.
03-177874, Aug. 1991. .
IBM Technical Disclosure Bulletin "Minimizing Hot Roll fuser
Elastomer Wear with Dual Hot Rolls" vol. 33, No. 4, Sep. 1990; pp.
71-73..
|
Primary Examiner: SMith; Matthew S.
Attorney, Agent or Firm: Baker & Botts, L.L.P.
Claims
We claim:
1. A fusing station for fixing multi-layer toner images on opposite
sides of a support material comprising first and second rollers
forming a pressure roller pair having a nip larger than 7 mm, the
first and second roller each making contact with toner images of
one side of said support material;
the first and second rollers each having:
a resilient covering for making contact at the nip with the toner
image on one side of said support material, and
means for heating the resilient covering of said roller, wherein
said first and second rollers have outer diameters which are
substantially equal, and wherein the thickness of the resilient
covering of the first roller is substantially equal to the
thickness of the resilient covering of the second roller, the
heating means of each roller having an individual power-control for
keeping the resilient covering of each roller at a substantially
constant temperature, with the temperature deviation between said
rollers being less than 20 K.
2. The fusing station according to claim 1, wherein the ratio (D1,
D2) of the respective outer diameters (D1, D2) of the first and
second rollers is between 0.9 and 1.1.
3. The fusing station according to claim 1, wherein said resilient
covering of each roller has a thickness larger than 1.5 mm,
preferably.
4. The fusing station according to claim 1, wherein each of said
rollers is coupled to a release agent applicator.
5. The fusing station according to claim 1, wherein each of said
rollers is coupled to a cleaning device.
6. The fusing station according to claim 1, wherein the resilient
covering of each roller is arranged for an advancement with a
peripheral speed synchronous to the advancement of the support
material through the fusing station.
7. The fusing station according to claim 1, wherein at least one
device which is in contact with a roller has a peripheral speed
synchronous to the advancement of the support material through the
fusing station, preferably with a mutual speed deviation less than
10%, more preferably with a speed deviation less than 2%.
8. The fusing station according to claim 1, wherein said
multi-layer toner image (8, 18) has dry toner particles.
9. The fusing station according to claim 1, arranged for movement
of said support material along a path between a toner transfer
station and the entrance of said fusing station, wherein said path
is substantially rectilinear.
10. A method for single pass fixing a duplex copy, said copy having
a toner image on both sides of a support material, using a fusing
station according to claim 1.
11. The method according to claim 10, wherein said toner image is a
multi-colour image composed of superimposed colour separation
images.
12. The method according to claim 10, further comprising the step
of preheating, acting substantially symmetrically on both sides of
the support material.
13. A method for fixing of double simplex copies in an
electrographic apparatus using a fusing station according to claim
1, characterised by the steps of
(i) using for a printing cycle two receptor sheets and conveying
them back to back in coinciding relationship along a common path
through said apparatus,
(ii) forming one toner image on one side of one receptor sheet and
a toner image on the opposite side of the other sheet while moving
both receptor sheets simultaneously through the apparatus thereby
to produce two simplex prints, and
(iii) fixing the toner images on both sheets.
14. The method according to claims 10 or 13, wherein the amount of
toner particles TM being deposited to reach maximum optical density
for black follows the equation
wherein TM is expressed in mg/cm.sup.2, d.sub.v50 is the average
volume diameter of the toner particles expressed in cm, and .rho.
is the bulk density of the toner particles in g/cm.sup.3.
15. The method according to claims 11 or 13, wherein the amount of
toner particles TM being deposited to reach maximum optical density
for each of the single colours yellow, magenta, cyan follows the
equation
wherein TM is expressed in mg/cm.sup.2, d.sub.v50 is the average
volume diameter of the toner particles expressed in cm, and .rho.
is the bulk density of the toner particles in g/cm.sup.3.
16. The fusing station according to claim 1 further comprising a
preheater, said preheater substantially symmetrically heating both
sides of the support material prior to entry into the nip.
Description
FIELD OF THE INVENTION
This invention relates to a fixing-system to be used within an
electrographic copying or printing apparatus capable of fusing, in
a single pass, toner material to both sides of a support member.
More in particular, it relates to a heat and pressure fusing of
electrographic multi-layer images on sheets.
BACKGROUND OF THE INVENTION
In a first kind of electrographic printing, particularly in the
process of electrophotography, a light image of an original
document to be copied or printed is recorded in the form of a
latent electrostatic image on a photosensitive member. The
generated electrostatic latent image is subsequently rendered
visible by application of electroscopic particles, commonly called
toner. The toner particles preferably have a definite electric
charge sign and as such are attracted by the electrostatic charge
pattern of opposite charge sign in proportion to the field strength
of the respective areas defining the pattern.
The toner particles forming the visual image are then transferred
from the photosensitive member to a support member or receptor
support, such as a sheet of plain paper or a plastic film, further
shortly indicated as "sheet". Since the toner image is then in a
loose powdered form which may be easily disturbed or destroyed, it
has to be permanently fixed or fused on said sheet in a fusing or
fixing device.
In a second kind of electrographic printing, particularly in Direct
Electrostatic Printing (DEP), electrostatic printing is performed
directly from a toner delivery means, e.g. a magnetic brush
assembly, on a receiving member substrate, called "sheet", by means
of an electronically addressable printhead structure. Herein, the
toner is deposited directly in an imagewise way on said sheet
without occurrence of any latent electrostatic image. An overall
applied propulsion field between the toner delivery means and a
receiving member support projects charged toner particles through a
row of apertures of the printhead structure. The intensity of the
toner-stream is modulated according to the pattern of potentials
applied to the control electrodes. The deposition step is followed
by a fusing step.
As a DEP device has already been described, e.g. in U.S. Pat. No.
3,689,935 (Pressman) and in EP-A-0 710 898 (Agfa-Gevaert N.V.), no
further description is necessary in the present application.
In order to permanently fix a toner image to a sheet, it is well
known in the art to apply thermal energy. By elevating the
temperature of the toner material to a point at which the
constituents of the toner coalesce and become tacky or melt, the
toner is absorbed into the fibres of the sheet or fixed to the
substrate. As thereafter the toner cools, solidification causes it
to be firmly bonded to the sheet.
Several approaches to thermal fusing of electroscopic toner images
are known from the prior art. Special attention has to be focused
on the production of duplex or recto/verso copies or prints, i.e.
copies where images are formed on both sides of the sheet.
The production of duplex or recto/verso copies poses problems due
to a severely occurring offset problem, which will be discussed in
great detail on the next pages.
Duplex printing in electrographic systems, e.g. in
electrophotographic copiers, working according to the two pass
method may be carried out in one of the following ways.
(i) A so-called "manual two pass method" that requires manual
re-feeding of multi-layer imaged simplex sheets, e.g. colour imaged
simplex sheets. That is, after the first side of a sheet is imaged
and fused, the sheet is transported to an output tray. Then, the
operator places this sheet back in one of the input trays, upon
which the sheet is again passed through the engine. This time an
image is transferred and fused onto the opposite side of each sheet
having an image on a first side.
(ii) A so-called "automated postponed two pass method", that
requires the collection of simplex sheets in a duplex tray. That
is, after the first side of a sheet is imaged and fused, the sheet
is transported to a duplex tray inside the engine. After the last
sheet in a set has been received in this duplex tray, all sheets
are again passed automatically through the imaging device. This
time an image is transferred and fused onto the opposite side of
each sheet having an image on a first side.
(iii) A so-called "automated immediate two pass method" that
requires reversing the simplex sheets immediately after fusing and
interleaving them with sheets receiving the first image on the
first side in order to receive an image on the opposite side.
These two-pass duplex methods have some very important drawbacks,
usually related to the twofold passing through the fuser.
(i) Two passes through the fuser require more energy than one pass.
This is especially important for the case of multi-layer imaging,
e.g. colour imaging, with its high energy requirement for thorough
fusing and mixing of the respective layers or colours.
(ii) At the same time the fuser needs to operate at twice the speed
of the duplex throughput, which again in the case of multi-layer or
colour fusing is not at all straightforward.
(iii) The change in moisture content (say about 30%) between the
first and the second imaging pass results in an image quality that
is not equal between the first side imaging and the duplex side
imaging.
(iv) In addition, this change in moisture content also alters the
mechanical properties of the paper, which--combined with the
additional complexity of a duplex paper path--results in a highly
increased risk for jams in duplex printing.
(v) Because of the need for a release agent (e.g. silicon oil) in
hot roller fusing, silicon oil remaining from the first pass colour
imaging may contaminate the image forming elements, resulting again
in non constant image quality over time, with possible effects such
as image smearing etc.
(vi) Excessive paper curl is not only troublesome in the processor
but also extremely difficult to handle in output stackers and
finishing devices.
In other prior art systems, also single pass duplex copying has
been disclosed. Three methods are known in the art.
(i) According to a first method, first and second images are formed
sequentially on a photoreceptor. The first image is transferred
from the photoreceptor to the first side of a receptor sheet. Then
the sheet is stripped off the photoreceptor, inverted while the
first image remains unfixed, and then the second image is
transferred to the second side of the receptor sheet. Both images
are then fixed onto the receptor sheet in a suitable fuser.
(ii) Other single pass duplex printing methods use intermediate
image carriers, e.g. a belt or a drum. The first and second images
are sequentially formed on a photoreceptor. The first image is
transferred to an intermediate image carrier. The receptor sheet is
then passed between the photoreceptor and the intermediate image
carrier. The receptor sheet is then simultaneously receiving first
and second images.
(iii) Other systems deal with "single pass duplex" methods
employing two photoreceptors and two exposure systems. A first
image is deposited on one photoreceptor and a second image is
deposited on the other photoreceptor. These systems are considered
the ultimate duplex throughput systems since they produce twice the
number of images of "two pass duplex" systems at equal process
speed.
Many problems exist with the traditional single pass duplex
systems.
(i) One problem is in conveying the duplex receptor sheet to the
fuser. In particular, the receptor sheet with the two unfused
images on opposite sides, must be transported from the toner
transfer station to the fuser. Preferably this is not done with a
conventional transport since the transport would make contact with
one of the sides of the receptor sheet and smear the unfused toner
image. Also, to avoid the leading edge of the sheet from downwards
deviating from the path between transfer station and fuser station,
it is preferred that this path is very short. Thereto, the fuser
must be very close to the photoreceptor. This creates problems in
mechanical mounting, problems due to unwanted heating the
photoreceptor and problems of contaminating the photoreceptor with
fuser release materials, e.g. silicone oil vapour.
(ii) In addition there is the problem of the rather uncontrollable
velocities of sheets passing through roller fusers. There seems to
be an obvious need to accurately match the velocity of the receptor
sheet transport with the velocity of the photoreceptor to prevent
"skips" and "smears" during transfer. Furthermore, for high
resolution digital printing, excessive instantaneous photoreceptor
velocity variations (cfr. "jitter") cannot be tolerated. Even in
conventional copiers it is preferable to keep the fuser rollers one
sheet length away from the transfer zone. For these reasons it is
desirable to thermally insulate and mechanically isolate the
photoreceptor transfer zones from the hot fuser rollers.
(iii) Single pass duplex systems using more than one photoreceptor
and more than one exposure system, generally require web paper feed
in which the copy is wound up on a roller or cut into individual
sheets after fusing. This, unfortunately, introduces additional
components and complexity into the system. It is, therefore, also
desirable to provide a single pass duplex system having a discrete
receptor sheet feed system rather than a web paper feed system.
(iv) Moreover, in high quality copying and printing it has to be
made sure that both sides of the duplex imaged sheets experience
substantially the same "fusing history", referring namely to the
temperature and pressure trajectory.
Multi-layer electrographic printing, e.g. multi-colour
electrophotographic printing, may seem equivalent to multiple
monochrome (commonly black and white) printing of various toner
layers. Yet, successive part images have to be recorded in
superposition. These successive part images may comprise a
superposition of different toner separation images. In one
embodiment, the traditional colour components cyan C, magenta M and
yellow Y, are augmented with at least one extra colour component
according to one toner type. This extra colour component may have
another density or colouring power (obtained by a different degree
of pigmentation) of either cyan, magenta or yellow. In another
embodiment, a traditional black component K is added to the three
usual colour components. In another embodiment, for each
traditional colour component, CMY or CMYK, at least a second colour
component, having a lower pigmentation level, C'M'Y'(K') is added.
According to another embodiment, some tone levels of the original
image are reproduced by applying two different toners, having
substantially the same chromaticity, or more specifically by
applying two achromatic toners, i.e. greyish or black toners of
which the chromaticity is substantially zero.
In one embodiment each single toner image is transferred to the
receptor sheet in superimposed registration, thereby creating a
multi-layered toner image on the receptor sheet. Thereafter, the
multi-layered toner image is permanently fixed to the receptor
sheet creating a multi-layer or colour copy or print. Whereas the
fixing of monochrome toner images does not raise major problems in
practice, the fixing of multi-layer or colour images is much more
difficult. We will base the discussion on colour images, which are
a specific case of multi-layer images.
(i) As a colour toner image intrinsically is thicker than a
monochrome toner image, for a same print-quality and a same
print-throughput, the supply of fusing heat has to be increased and
even controlled more stringently.
(ii) The increased amount of toner requires a longer fusing time
demanding a nip with a larger length or a slower rotation of the
fuser rollers. It may be remarked that the nip between both
rollers, more exactly between the resilient coverings of these
rollers, is in fact the area where heat and pressure initiate the
fusing and thus the fixing of the toner image on a sheet conveyed
between the rollers.
(iii) The fixing of multi-layer images is also difficult as
compared to the prior art of fixing single layer images, in that it
needs a strongly different geometry of the fixing rollers, calling
for a dedicated design of the kind and the geometry, e.g. thickness
of the resilient layer on each roller, the diameter of the rollers,
the pressure applied to the rollers, etc.
In view of the many problems described, a very interesting
application comprises U.S. Pat. No. 4,427,285. However, some
drawbacks still pose severe restrictions to the effective use of
said patent.
A first restriction of the solution disclosed in U.S. Pat. No.
4,427,285 is that it is not intended for and hardly can be applied
for fusing multi-layer toner images.
A second and important restriction of U.S. Pat. No. 4,427,285 is
that its solution needs heat isolation means between the fusing
station and the photoreceptor.
Hereto, it discloses e.g. a transport mechanism for conveying a
receptor sheet having toner images on both sides, towards the heat
source for fusing, thereby thermally isolating the photoreceptor
from the heat source.
U.S. Pat. No. 4,427,285 also discloses a heat shield disposed
between a transfer station and a heat applying device, thereby
carrying out two distinct functions, namely
(i) isolating the heat, and
(ii) tacking the unfused images onto the receptor sheets.
More particularly, it discloses the use of compacting rollers,
which have to fulfil both said functions of thermal isolating and
tacking.
As will be clear from the detailed description, it is a remarkable
advantage of the present invention that no initial tacking down is
necessary and that no compacting rollers are necessary.
It will also become clear from the detailed description, that no
intermediate fusing is necessary. Such an intermediate fusing
inevitably would increase the construction-cost of the apparatus,
and could reduce the reliability of the system, as the dimensional
stability of the sheets would diminish because of changing moisture
content.
In view of the above, fusing stations of the type described above
are unsuitable for being installed in electrographic apparatus
designed for single-pass fusing of sheet-fed multi-layer or colour
duplex copies.
OBJECTS OF THE INVENTION
It is an object of the present application to provide an apparatus
and a method providing good fusing quality for single pass duplex
copies of sheet-fed multi-layer copies without intermediate
fusing.
Further objects of the present invention will become clear from the
description given hereinafter.
SUMMARY OF THE INVENTION
The above mentioned objects are realised by the specific features
according to claim 1.
Specific features for preferred embodiments of the invention are
set out in the dependent claims.
Further advantages and embodiments of the present invention will
become apparent from the following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described hereinafter by way of example with
reference to the accompanying drawings.
FIG. 1 is a diagrammatic view of a fusing station according to the
current invention, comprising a pressure roller pair;
FIG. 2 is a diagrammatic view of another fusing station according
to the current invention, comprising a flow of hot air.
DETAILED DESCRIPTION OF THE INVENTION
The fusing station according to the present invention will be
described hereinafter and illustrated by means of the accompanying
figures, which are not intended to restrict the scope of protection
applied for by the present application. In the following drawing
and description, like referrals (e.g. 1 and 11) constitute like
parts (e.g. fixing rollers) with like operation.
FIG. 1 gives a schematic cross-sectional representation of a fusing
station according to the present invention.
As an aid to a better understanding of the specification and the
claims to follow, the meaning of some specific terms are explained
first.
The terms "support material", "receptor support", "support or
substrate member", "receptor sheet" or shortly "sheet" as used
further in the present specification stand for a sheet of opaque
paper, a white bond paper, a resin coated paper, a transparent
film, a plastic, a laminate of both, an adhesive label and the like
onto which the transferred image is received. This sheet may be an
end-product as such but it may also form an intermediate step in a
reproduction process. For example, it may be used, after a suitable
treatment, as a so-called transfer element, e.g. as a printing
plate for printing images by planographic printing techniques onto
a final support. Many experiments carried out by the inventors
related to sheets of a so-called "1001 paper", having a specific
weight of about 100 g/cm.sup.2.
The term "colour" is not strictly limited to the development of
usual colour separation images by conventional magenta, cyan and
yellow and optionally also black toners (abbreviated as CMY or
CMYK). It encompasses also the production of images by means of
less or more than three colours; by means of different shades of
one colour, e.g. different grey shades, or even multiple layers of
one toner; the covering or coating of an image by an image-wise
applied transparent, coloured, fluorescent or otherwise treated
varnish, and the like.
The term "printing" stands in the first place for a printer which
creates an output printing image by laying out the image in a
series of horizontal scan lines, each line having a given number of
pixels or picture-elements per inch. An exposure station for
exposing the recording may comprise a laser with a rotating mirror
block, a LED array, a uniform light source and a plurality of
individually controllable light valves, an arrangement with
deformable micro-mirror devices (DMD), etc. However, the term
printing encompasses also an apparatus in which the exposure of the
recording member occurs by the optical projection of an integral
image, such as in a copier. Further, the term printing also
encompasses DEP-devices.
A general overview of an electrographic copying or printing
apparatus capable of providing colour images on both sides of
sheets of paper is given in pending patent application
EP-A-96.203.561.4, entitled "Electrostatic colour printing
apparatus" (in the name of Agfa-Gevaert N.V.). In said application,
an electrostatographic colour printing apparatus is described which
comprises exposure units for forming successive electrostatic
colour part images on both surfaces of a recording member in the
form of an endless belt. The application addresses developing
stations for sequentially developing such electrostatic latent
images to form toner images on such belt, and electrostatic
transfer stations for sequentially transferring the toner images
from such belt in superposition onto a receptor sheet fed through
the transfer stations while the receptor sheet is in contact with a
belt section to produce a multi-colour duplex image.
FIG. 1 of the instant application shows an embodiment of a heat and
pressure fusing device 25, the construction of which is described
below. Fusing station 25 comprises a pair of rollers 1, 11. Each
roller comprises a solid of revolution made of heat conductive
materials 3, 13, e.g. a cylindrical aluminium core or tube. Both
heat conductive solids preferably have substantially equal
diameters, and are mounted for rotation about their axis by means
known in the art. Their peripheral surfaces are provided with a
resilient covering 2, 12 of non-adhesive material, e.g. silicone
rubber. The resilient covering preferably may be coated with a
tetrafluoroethylene resin, a fluorocarbon resin or the like.
Both rollers 1 and 11 may be provided with an internal heating
source 4, 14 such as a tubular infrared lamp.
The fusing device comprises means for urging the rollers 1, 11
against each other. As such, a nip is formed with an appropriate
length. In our experiments a nip length of about 9 mm was highly
preferred. Through the nip a sheet 9 having non-fixed or partially
fixed thermoplastic powder or toner images 8, 18 deposited thereon
is passed for fixing the toner to the sheet. The urging means may
comprise a spring or a pneumatic mechanism (not shown in FIG.
1).
In the vicinity of rollers 1 and 11 there may be provided means 5,
15 for coating an inhibitor solution, release agent, or oil onto
the rollers. This prevents toner offset for an easy release of a
sheet 9 from the rollers 1, 11. In addition, stripping means (not
shown) or the like may be provided for ensuring a reliable release
of the sheet from rollers 1, 11.
After having disclosed the basic construction of fusing station 25,
now its functional operation will be described. As can be seen in
FIG. 1, the sheet 9 bearing toner images 8, 18 on both surfaces is
passing through the fusing station 25. The outer surfaces of the
fixing rollers, contacting the sheet of support material 9, move
with a peripheral velocity synchronous to the speed of advancement
of said sheet of support material 9 through the fusing station
25.
As has been put forward hereinbefore, it is important to ensure
that no toner particles are offset from the sheet 9 to the rollers
1, 11, and vice versa, neither by friction, neither by
adhesion.
Now in order to ensure that no offset due to friction between said
sheet of support material 9 and the rollers 1, 11 would occur, said
rollers 1, 11 are preferably driven by a suitable motor and
suitable belts or gears (not shown) so that the outer surfaces of
said rollers 1, 11 advance synchronously to the advancement of the
sheet of support material 9 through the fusing station 25. In this
way no offset due to friction occurs between said rollers 1, 11 and
said sheet of support material 9.
In order to prevent any offsetting of toner from said sheet of
support material 9 to rollers 1, 11 due to adhesion of toner
particles to said rollers, it is known for those skilled in the
art, to cover the roller with a surface layer or resilient covering
2, 12 of a release material such as poly-tetrafluoroethylene,
silicone rubber or the like.
As these materials are heat insulators, the thickness of the layer
of these materials on the roller must be kept thin since heat
conductance decreases with increased thickness.
It may be repeated here that in a type of fusing station using a
pair of heated rollers 1, 11 through which the sheet 9 passes, said
heated rollers are preferably covered with a release material or
resilient covering 2, 12 and an additional release agent 5, 15 such
as silicone oil is preferably used to reduce the offset problem due
to the adhesion of toner material to said heated rollers.
In a heated roller pair contact fusing station, intimate contact
between the sheet 9 and the heated roller pair 1, 11 is essential
for an effective fusing of the toner material 8, 18 on the sheet 9.
Indeed, all the heat needed for fusing the toner material has to be
passed on to said material through heat conductance from the heated
rollers to said toner material. This implies that, during fixing,
the toner being fused will be in direct contact with the heated
rollers and simultaneously subjected to pressure.
In practice the temperature of the rollers 1, 11 may be kept
substantially constant at a predetermined value by introducing a
thermistor, e.g. a bimetal within said roller or, more preferably,
by a temperature detecting element provided near the surface of the
roller, and connecting said thermistor to a thermostatic control
circuit (not shown). Even more than one temperature sensor may be
used, preferably situated on different positions relative to the
roller. For example, one temperature sensor can be in rolling
contact with the resilient covering 2, 12 of a fixing roller 1, 11
within the image zone, and another temperature sensor can be in
contact with the same fixing roller but outside the image zone.
Also a contactless temperature sensing is highly preferred for
measuring the temperature of the surface of the rollers 1 and 11,
especially within the image zone.
As the sheet of support material 9 leaves the fusing station 25, it
may be taken by an additional pair of rollers (not shown) for
further transport to a copy paper tray and for subsequent
removal.
In short, a first embodiment of a fusing station 25 according to
the present invention is disclosed for use in an electrographic
apparatus; comprising as well electrophotographic (comprising an
electrical photoconductor), electrophoretic (referring to toner
images formed by liquid toner particles), as electrostatic (e.g.
DEP-devices) apparatus.
A multi-layer toner image is fused to a support material in
sheet-form 9 while said sheet 9 is moved over a predetermined path
7. The fusing station comprises two heated fixing rollers 1 and 11,
each for rotating in contact with one side of the sheet. A driving
means may be used to rotate the fixing rollers. The outer surface
of both rollers is moving synchronously with the speed of
advancement of the sheet 9. Pressing means applies an urging force
on said fixing rollers 1 and 11. The heating sources 4 and 14 have
substantially identical characteristics (geometry, spectrum, power
. . . ) and are preferably radiant. Both fixing rollers each have a
resilient covering 2 and 12, which by the pressure between both
rollers forms a heating nip. As such, a symmetrical fixing
operation on both sides 8 and 18 of said sheet is provided by said
fixing rollers which preferably have a substantially equal
construction and are positioned substantially symmetrically to the
path 7 of the sheet.
In a preferred embodiment according to the present invention the
thickness t1 of the resilient covering 2 of the first roller 1 is
substantially equal to the thickness t2 of the resilient covering
12 of the second roller 2. More particularly, the ratio (t1/t2) of
the thicknesses (t1 and t2) of the resilient coverings 2, 12 of the
respective fixing rollers, which exemplary are about 2.5 mm, is in
a range between 0.9 and 1.1; more preferably between 0.95 and
1.05.
In a further preferred embodiment according to the present
invention the ratio (D1/D2) of the diameters (D1 and D2) of the
outer circumferences of the respective fixing rollers, which
diameters D1 and D2 exemplary are about 73.5 mm, is in a range
between 0.9 and 1.1; more preferably between 0.95 and 1.05.
In a further preferred embodiment according to the present
invention the nip created between the fixing roller and the
pressure roller should have a length larger than 3 mm, preferably
larger than 5 mm, and more preferably larger than 7 mm.
In a further preferred embodiment according to the present
invention a separate power-control controls each heating source
such that the outer circumferences of both fixing rollers have a
substantially equal temperature; say e.g. about 443 K (or
170.degree. C.).
In a particularly preferred embodiment, the urging force without
sufficient heating is not sufficient to produce fusing, without
offset, at said predetermined speed; which may be about 95
mm/s.
A fusing device according to the present invention may comprise
means for treating the surface of the fixing roller to release a
fixed sheet more easily. Stripping of a fixed sheet may be done by
means of release agent, e.g. oil, applied to the fixing roller, but
also by means of mechanical or pneumatic systems. A system for
fusing a toner image on a sheet then comprises heated fixing
rollers exerting a pressure on at least one portion of a toner
image on the sheet by a nip formed by pressure between the fixing
rollers. Preferably, it further comprises an oil application system
for application of oil to the fixing rollers.
More in particular, in a further preferred embodiment according to
the present invention, said fusing station further comprises
release agent applicators or oiling devices 5 and 15, allocated
individually to each fixing roller. These oiling devices have a
construction, a position relative to the fixing rollers and an
individual oiling control such that the outer circumferences of
both fixing rollers receive a substantially equal layer of release
agent.
In a further embodiment according to the present invention, said
fusing station also comprises cleaning devices 6 and 16, allocated
individually to each fixing roller. These cleaning devices
preferably have a construction, a position relative to the fixing
rollers and an individual drive control such that the outer
circumferences of both fixing rollers are cleaned substantially
equally.
In a further preferred embodiment, each of said heating sources
comprises an infrared or a halogen quartz lamp, mounted
individually within each fixing roller.
In still another embodiment according to the present invention, one
lamp or a plurality of lamps is mounted within each roller.
In another embodiment according to the present invention, a
resistive heater may be used to heat the heat conducting core
3,13.
In a further preferred embodiment of a fusing station according to
the present invention, said fixing roller is made of a suitable
heat conducting core 3 and 13 and is resiliently covered with a
suitable surface layer of a deformable material 2 and 12. More
particularly, the outer surface of the fixing roller is covered
with a suitable surface layer of a deformable material or resilient
covering, which preferably comprises at least
(i) an inner layer of a soft or elastically deformable and thermal
conductive rubber, and
(ii) an outer layer of a release material.
Preferably, said heat conducting core 3 and 13 is made of copper,
of aluminium, or an alloy of one of these materials. A thickness of
e.g. 4,25 mm has been preferred in the experiments carried out by
the inventors.
In a further preferred embodiment of said fusing station, said
resilient covering 2 and 12 is silicone rubber or a
fluor-elastomer. In the experiments carried out by the inventors a
thickness of e.g. 2,5 mm silicone rubber with a hardness of 40
Shore has been preferred.
In a fusing station according to the present invention, a thermal
sensor, e.g. a thermistor, is connected to a thermostatic control
circuit, the temperature of the roller is kept substantially
constant at a predetermined value, said value being set between the
temperature at which the resinous toner powder becomes tacky or
melts and the fusing temperature of said toner. Preferably, each
heating means has an individual power-control for keeping the
resilient covering of each roller at a substantially equal
temperature, the temperature deviation between said rollers being
less than 20 K, preferably less than 5 K.
In a fusing station according to a further preferred embodiment the
outer surface of the resilient covering 2 and 12 of the fixer
rollers advances synchronously with the advancement of the sheet 9
through the fusing station 25 and at least one of the devices e.g.
sensor, cleaning, release agent applicator which are in contact
with the fixing rollers 1 and 11, have a synchronously rolling
contact.
It is further highly preferable that in a fusing station 25
according to the present invention, said path 7 of the sheet, at
least between the transfer station and the nip, is substantially
rectilinear. Between a transfer station and the nip of the rollers
of the fusing station, a radius of curvature of said path
preferably is larger than two times the outside diameter of the
rollers, more preferably larger than five times.
In order to obtain a good and equal thermal behaviour of both
fixing rollers, they preferably comprise substantially the same
materials as well for the core 3, 13 as for the resilient covering
2, 12, in substantially same thicknesses, etc. Further, said fixing
rollers preferably are mounted with their longitudinal axes in
parallel. Generally, both fixing rollers have a same geometry,
mostly being cylindrical. Nevertheless, convex and/or concave
geometries of the fixing rollers (e.g. in order to prevent possible
wrinkling of the sheets) also fall within the scope of the present
invention.
Apart from a physical fusing station as disclosed before, also a
method is disclosed for single pass fixing of duplex or recto/verso
copies comprising toner images 8 and 18 on both sides of a sheet 9
using a fusing station 25 as described above.
In a further preferred embodiment of a method according to the
present invention, toner image 8 and 18 is a multi-layer image
composed of superimposed colour separation images.
In a particular method according to the present invention, the path
7 of the sheet 9 of support material is substantially horizontal.
By the wording substantially horizontal is meant a path within a
range of [-5.degree., +5.degree.] to a horizontal path.
In another particular method according to the present invention,
the path 7 of the sheet 9 of support material is substantially
vertical. By the wording substantially vertical is meant a path
within a range of [-5.degree., +5.degree.] to a vertical path.
Some advantages of a horizontal path comprise:
(i) if the sheets in an input paper tray and in an output paper
tray lay in a horizontal position, said sheet can follow a
rectilinear path, which is very advantageous for a high reliability
of the transport system (e.g. a very low risk for paper jam and for
wrinkles);
(ii) the height of the apparatus can be rather low, which may be
extra comfortable for the operator.
Some advantages of a vertical path comprise:
(i) the operations acting on the sheet may be carried out with a
high symmetry, because there is no preferential influence from heat
or gravity as it regards both sides of the sheet;
(ii) the floor-space necessitated for the apparatus can be rather
small.
Yet any other orientation of the path 7 of the sheet 9 may be
advantageous and is included within the scope of the present
invention.
A further preferred method comprises a preheating step, acting
symmetrically on both sides 8 and 18 of the blank sheets 9, thus
before said sheets receive any toner particles. By doing so, some
mechanical characteristics of the sheets (e.g. moisture contents or
differences thereto) may be equalized, so that possibly a still
lower jam rate and even a better fusing quality can be
attained.
As will be clear from the background section of this specification,
single pass duplex multi-layer toner fusing on sheets nowadays
presents some other difficulties to be solved.
Amongst them:
(i) transporting the duplex powdered sheets from the duplex imaging
device towards the single pass duplex fuser without damaging the
non-fixed images;
(ii) providing a specific fusing speed required to obtain stable
image quality on a wide variety of base print materials, whereas
the imaging portion of the engine usually only has a very limited
number of discrete imaging speeds;
(iii) moreover, the fusing and imaging speed can hardly be made
exactly equal, thereby necessitating a way to decouple both
speeds.
In a method according to the present invention, these just
mentioned difficulties are solved by providing a buffering device
between imaging station or transfer station and fusing station.
This buffer can handle differences in speed, vibrations, etc.
The purposes of the just mentioned buffer may be explained more in
detail as follows. Fuser station 25 melts the toner images 8, 18
transferred to the sheets 9 in order to affix them. It will be
understood that this operation requires a certain minimum time,
since the temperature of the fuser is subject to an upper limit
which must not be exceeded. Otherwise the roller lifetime becomes
unsatisfactory. In other words, the speed of fuser station 25 is
limited. The speed of the image formation stations (not shown), on
the other hand, is in principle not limited for any particular
reason. On the contrary, it is advantageous to use a high speed of
image formation and image transfer, since the (e.g. four) colour
separations of each colour image are preferably written by an
exposure station in succession. This means that the recording time
of one colour image amounts to at least four times the recording
time of one part image. All this means a relatively high speed of
the photoconductive belts, and thus of the synchronously moving
sheets, as compared with a maximum usable travelling speed through
the fuser station. In order to indicate some practical
test-results, in an apparatus according to the present invention,
the speed of the photoconductive belts amounted to 295 mm.s.sup.-1,
whereas the fusing speed was 100 mm.s.sup.-1.
Further, it may be desirable to adjust the fusing speed
independently from the image processing speed, for obtaining
optimum results. It should be noted that the image processing speed
in the imaging stations is preferably constant.
The length of the buffer station needs to be sufficient large for
receiving the largest sheet size to be processed in the
apparatus.
Whereas the buffer station operates initially at the speed of the
photoconductive belts, the speed of this buffer station is reduced
to the processing speed of fuser station 25 as the trailing edge of
the sheet has left the image forming station.
As disclosed in European Patent Application n.sup.o 96.200.977.5
(in the name of Agfa-Gevaert N.V.), in a colour toner image, the
amount and/or the dispersion of pigment in the toner particles, for
a single colour, is preferably adjusted such that a full saturated
density in said colour is achieved by the deposition of a thin,
almost single, layer of toner particles. By doing so the gloss
differences, due to (possibly great) differences in the height of
the various layers of deposited toner particles, are minimized.
In a preferred embodiment, the amount of toner particles per unit
area (Toner Mass, TM) being deposited to reach maximum optical
density for each of the single colours follows the equation:
wherein TM is expressed in mg/cm.sup.2, d.sub.v50 is the average
volume diameter of the toner particles expressed in cm), and .rho.
is the bulk density of the toner particles in g/cm.sup.3 (e.g.
.rho.=1.1 to 1.3 g/cm.sup.3).
In this application by maximum optical density for each of the
single colours is meant an optical density on a reflecting support
between 1.4 and 1.6 for yellow, magenta and cyan and an optical
density between 1.6 and 2.0 for black.
Thus, in the production of full-colour images, e.g. with four
colour toners YMCK, each of the toners having a d.sub.v50
=8.10.sup.-4 cm and a density of 1.25 g/cm.sup.3, the very darkly
coloured areas will be formed by the overlay of about 2,5 layers,
each being made up by 0.8 mg/cm.sup.2 of toner. Fixing of a
resulting toner layer of about 2,5 mg/cm.sup.2 is quite difficult
and requires special measures.
Now, we just have disclosed an apparatus and a method for single
pass fixing of a multi-layer toner image toner image to a sheet of
support material. Also disclosed was a method particularly suitable
for fixing duplex copies.
In a further embodiment of a method according to the present
invention, the amount of toner particles TM being deposited to
reach maximum optical density for black (i.e. an optical density
between 1.6 and 2.0 on a reflecting support) follows the
equation
wherein TM is expressed in mg/cm.sup.2, d.sub.v50 is the average
volume diameter of the toner particles expressed in cm, and .rho.
is the bulk density of the toner particles in g/cm.sup.3.
In another preferred embodiment of a method according to the
present invention, the amount of toner particles TM being deposited
to reach maximum optical density for each of the single colours
yellow, magenta, cyan (i.e. an optical density between 1.4 and 1.6
on a reflecting support) also follows the same equation
wherein TM is expressed in mg/cm.sup.2, d.sub.v50 is the average
volume diameter of the toner particles expressed in cm, and .rho.
is the bulk density of the toner particles in g/cm.sup.3.
In case the image is developed by means of a colourless toner as
exemplified in, e.g., EP-A 0 656 129, EP-A 0 629 921, EP-A 0 486
235, U.S. Pat. No. 5,234,783, U.S. Pat. No. 4,828,950, EP-A 0 554
981, WO 93/07541 and Xerox Research Disclosure Journal, Vol.16,
N.sup.o 1, p. 69 (January/February 1991), this colourless toner is
preferably deposited in an amount TM fulfilling the equation [1].
Also in this case, the present invention remains applicable.
APPLICABILITY
A contact heat and pressure fixing according to the present
invention is more advantageous than fixing by utilising irradiated
heat in that it needs less electric power, and in that the danger
of fire hazard and burning of the sheets is much lower.
It is a remarkable further advantage of the present invention to
obtain an equal quality on both image-sides, even when some
characteristics of the system might be different, e.g. different
roughness on the recto versus the verso side of the sheets,
different construction or position of the release agent
applicators, thermal influences differing on both fixing rollers,
etc.
For the purposes of the present invention the latent electrostatic
image may be formed by an exposure of an electrostatically charged
photosensitive member to a light image of an original document. Or,
the latent electrostatic image may be generated by exposing the
photosensitive member to a plurality of appropriately activated
discrete spot-like sources of radiation. Said discrete spot-like
sources of radiation may be constituted by a linear array of light
emitting diodes (LED's) or by a laser, the beam of which is
modulated to determine during each scan movement a plurality of
elementary image sites that may receive radiation or not depending
on the modulation of the radiation beam.
Evidently, a method for single pass fixing of simplex copies
(comprising toner images on one side of a sheet) using a fusing
station 25 according to the present invention, also falls within
the scope of protection.
The present invention also may be used in a method for producing
double simplex copies or prints by means of a single pass duplex
copier or printer.
Said method is characterised by the steps of
(i) using for a copying or printing cycle two receptor sheets and
conveying them back to back in coinciding relationship along a
common path through said printer,
(ii) forming one toner image on one side of one receptor sheet and
another toner image on the opposite side of the other one while
both receptor sheets are simultaneously moved through the printer
thereby to produce two simplex prints, and
(iii) fixing the toner images on both sheets. For more specific
information, reference is made to patent application
EP-A-96.203.558.0 (in the name of Agfa-Gevaert N.V.).
In a preferred embodiment it is desirable to provide a single pass
duplex system having a discrete receptor sheet feed system.
Optionally also a web paper feed system may be used with the
concept of a symmetrical fixing operation as laid down in the
present application.
Apart from traditional toner images formed by dry toner particles,
the present invention also may be carried out on toner images
formed by liquid toner particles, e.g. applied by
electrophoretics.
As also mentioned in the introduction of this specification, the
use of a fixing device according to the present invention is
particularly interesting for the fusing of electrographic
multi-layer images, e.g. electrophotographic colour images, even
for simplex or single-sided copies.
However, its use is still more interesting in the fusing of duplex
colour images since the problem of surface temperature fluctuations
of fixing rollers is even more stringent in such application. In
this connection, we refer to our above mentioned co-pending
EP-A-96.203.561.4.
It may be clear for people skilled in the art, that the previously
mentioned buffering device between imaging and fusing can be used
advantageously also in other types of fusing stations, as e.g. in
fusing stations using directly radiating radiators (thus not being
built in rollers) as short-wave (e.g. infrared lamps), mid-wave or
long-wave radiators (e.g. resistive or ceramic elements) or flash
lamps, in fusing stations using electromagnetic waves (e.g.
micro-waves), in fusing stations using hot air, etc.
In an alternative embodiment according to the present invention,
the symmetrical fixing operation can also be realised by using a
hot air fusing station. Herein the fusing is done nearly
contactless (meaning that substantially no rollers nor plates are
in contact with the sheets while being fixed) and a controlled
stream of hot air is conveyed symmetrically at both sides of the
sheet. Evidently, flow and temperature of said hot air have to be
controlled within acceptable limits.
Such a fusing method comprises a step of moving the sheet, which
may be carried out by different mechanisms. For example: gravity as
such in case of a vertical path, downwards oriented, of the sheet,
a belt, a clamp mechanism gripping the sheet on non-imaged borders,
or another transporting means comprises means for keeping a fixed
orientation of the sheet and means for keeping contact with an edge
of the sheet, etc.
Such a fusing method also comprises a simultaneous step of sheet
heating, while moving said sheet, by symmetrically applying hot
air, which may be carried out by different mechanisms. For example:
by a set of two perforated plates, localised at both sides of the
sheet on the path followed by the sheet(see e.g. FIG. 2).
Said hot air fulfils two different functions:
(i) an air-cushion function which helps the contactless moving of
the sheet,
(ii) a fusing function by symmetrically and homogeneously heating
both sides of the sheet.
In short, a further preferred embodiment of a method for single
pass fixing of duplex or recto/verso copies of resinous powder
colour images to a support material, comprises the simultaneous
steps of
(i) moving said sheet via a predetermined path through a fusing
station,
(ii) applying hot air in said fusing station to both sides of said
sheet, wherein said fusing station comprises heating sources with
substantially identical operational characteristics, and wherein
said application of hot air is characterised in that a symmetrical
fixing operation on both sides of said sheet is provided by heating
said hot air to a substantially equal temperature and by enforcing
said hot air in a substantially equal flow to both sides of said
sheet. It may be remarked that this embodiment also can be applied
to receptor support materials which are not separate sheets in the
strict meaning, but which are in web-form.
Various modifications will become apparent to those skilled in the
art based on the teachings of the present disclosure, without
departing from the scope thereof.
Among these modifications, sheets fed from the input-stack (not
shown in FIG. 1) can occasionally be subjected to a drying
operation prior to the toner image transfer, in order to get a
sufficiently low moisture content, e.g. below 60%.
Another modification also protected by the present application,
comprises a preheating step acting on the blank sheets prior to the
fusing step, even prior to the transfer step or even prior to the
development step. Although such a preheating increases the
construction-cost of the apparatus, the operation-cost of the
apparatus decreases; as the fixing energy in the fixing step
decreases, the change of moisture in the sheets decreases, the
possible jam rate decreases.
______________________________________ Parts list
______________________________________ 1, 11 (fixing) rollers 2, 12
resilient covering 3, 13 heat conducting core 4, 14 heating sources
5, 15 release agent applicator (oiling devices) 6, 16 cleaning
devices 7, 17 path 8, 18 toner material 9 sheet(s) 25 fusing
station ______________________________________
* * * * *